Automotive bumpers and like plastic substrates are generally coated by spray coating, such as air spray coating or airless spray coating. In recent years, however, electrostatic coating, which shows excellent deposition efficiency and thus discharges only a small amount of coating material to the environment, has been finding wider application.
Since plastic substrates generally have high electrical resistance (usually about 1012 to about 1016 Ω/sq.), it is extremely difficult to electrostatically apply a coating material to plastic substrate surfaces directly. Therefore, before electrostatic application, conductivity is imparted to plastic substrates themselves or their surfaces so that the substrates have a surface electrical resistance below 109 Ω/sq.
For example, before electrostatic application of a coating material to a plastic substrate, a conductive primer may be applied to impart conductivity to the substrate. A coating material containing a resin component and conductive filler is conventionally used as such a conductive primer.
Particles of conductive carbon, metals, conductive metal oxides, etc. have been heretofore used as conductive fillers. The form or shape of such conductive filler particles is usually a powder, needles, fibers, spheres or the like.
When a carbon powder or carbon fibers are added to a coating material as a conductive filler, although a relatively small amount can impart conductivity, the resulting coating layer has reduced whiteness, i.e., reduced brightness, and thus affects the color properties, such as brightness, of the upper coating layer to be formed thereon.
Metal powders have high conductivity, but need to be added in large amounts, since the particles of metal powders need to be in contact with one another to form an electrical conduction path in a coating layer. Thus, use of a metal powder as a conductive filler impairs the whiteness of the coating layer and stability of the coating material.
Japanese Examined Patent Publication No. 1994-17231 discloses, as a conductive filler that does not decrease the brightness of the coating film, a white conductive titanium oxide filler comprising high-quality titanium oxide particles having on their surfaces a conductive layer comprising tin oxide and antimony oxide. However, since recently there are concerns about toxicity of antimony, such a conductive filler is not preferable.
Japanese Patent No. 3357107 discloses, as a conductive filler for coating materials, a white conductive titanium dioxide powder comprising titanium dioxide particles having on their surfaces a tin oxide covering layer containing 0.1 to 10 wt. % of phosphorus. The powder contains no antimony and thus causes no toxicity problems, but has a problem in that it is difficult to a form a layer with good conductivity on the titanium dioxide particle surfaces. Therefore, coating layers formed from coating materials containing the powder as a conductive filler do not have stable conductivity, and thus cannot be electrostatically coated in some cases.